1. Field of the Invention
The present invention generally relates to a torque converter. More specifically, the present invention relates to a torque converter having a lock-up device with an elastic member.
2. Background Information
A torque converter includes a torus having three types of vane wheels (an impeller, a turbine and a stator) and serves to transmit torque by a fluid in the torus. The impeller forms a fluid chamber filled with operating oil with the front cover. The impeller mainly includes an annular impeller shell, a plurality of impeller blades fixed to an inner surface of the impeller shell, and an annular impeller core fixed to the opposite ends of the impeller blades. The turbine is disposed in the interior of the fluid chamber opposite the impeller in the axial direction. The turbine mainly includes an annular turbine shell, a plurality of turbine blades fixed to a surface of the turbine shell facing the impeller, and an annular turbine core fixed to the opposite ends of the turbine blades. A radially inner portion of the turbine shell is fixed to a flange of a turbine hub by a plurality of rivets. The turbine hub is unrotatably connected to a main drive shaft of a transmission. The stator regulates the direction of the operating oil that is returned from the turbine to the impeller. The stator is disposed between radially inner portions of the impeller and the turbine. The stator mainly includes an annular stator shell, a plurality of stator blades formed on an outer circumference of the stator shell, and an annular stator core formed on the opposite ends of the stator blades. The stator is supported by a stator shaft via a one-way clutch.
One of coefficients expressing performance of the torque converter is capacity coefficient C, which is given by a formula (1) as shown below.C=TI/nI2  (1)
Capacity coefficient C is a coefficient showing a relationship between nI and TI, wherein nI is the number of revolutions of the input shaft of the torque converter and TI is a torque input to the input shaft of the torque converter. Capacity coefficient C represents a torque that can be input at a certain number of revolutions. As understood from the aforementioned formula, the larger the capacity coefficient C of the torque converter is, even if the number of revolutions nI of the input shaft of the torque converter, i.e., the number of revolutions of the engine, remains the same, the larger torque TI that can be input to the torque converter is. This means that the larger the capacity coefficient C of the torque converter is, even if the number of revolutions nI of the input shaft of the torque converter remains the same, the larger the load to the engine is. In a range where the speed ratio, which is a ratio at a rotation speed of the turbine relative to a rotation speed of the impeller, is small, i.e., in an idling range of the engine and a range close to the idling range, the capacity coefficient C is large. Further, as the speed ratio increases, i.e., as the number of revolutions of the engine rises, the capacity coefficient C decreases.
If the capacity coefficient C at a high speed ratio is improved in the torque converter, an acceleration performance during an intermediate acceleration of a vehicle is improved. Therefore, it is a common practice to increase the capacity coefficient C in a high speed ratio range, as shown in Unexamined Japanese Patent Publication JP2002-106676.
The lock-up device is disposed in a space between the turbine and the front cover. The lock-up device is adapted to couple mechanically the front cover to the turbine to transmit directly torque from the front cover to the turbine. The lock-up device includes a disc-like piston to be pressed against the front cover, a retaining plate fixed to a radially outer portion of the piston, torsion springs supported by the retaining plate in the rotational direction and radially outward, and a driven plate supporting both the ends of the torsion springs in the rotational direction, with the driven plate being fixed to a turbine shell or the like of the turbine.
When the lock-up device is engaged, torque is transmitted from the front cover to the piston, and then to the turbine via the torsion springs. Further, when torsional vibration is input to the lock-up device, the torsion springs are compressed between the retaining plate and the driven member in the rotational direction to absorb and attenuate the torsional vibration. An example of this structure can be found in Unexamined Japanese Patent Publication H03-10455.
In recent years, lock-up devices employing a multi-plate clutch having a plurality of friction surfaces to address an increase in the engine torque have been used. There is a known torque converter in which torque is transmitted by fluid only when a vehicle starts, and the lock-up device is connected at a speed of 10 km per hour or greater, for example. In such a structure in which a lock-up region is expanded, improved performance of the torsion spring is desired to absorb and damp torsional vibrations sufficiently in response to torque changes from the engine.
As described before, it is necessary to enlarge the axial dimension of the lock-up device to realize a multi-plate structure or to improve the performance of the torsion springs. However, since the torsion springs are disposed axially between the front cover and the turbine, the size of the entire torque converter increases if the torsion spring size is increased.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved a torque converter. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.